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Internet Engineering Task Force (IETF) M. Thomson
Request for Comments: 8188 Mozilla
Category: Standards Track June 2017
ISSN: 2070-1721
Encrypted Content-Encoding for HTTP
Abstract
This memo introduces a content coding for HTTP that allows message
payloads to be encrypted.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc8188.
Copyright Notice
Copyright (c) 2017 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Requirements Language . . . . . . . . . . . . . . . . . . 3
2. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . . . 3
2.1. Encryption Content-Coding Header . . . . . . . . . . . . 5
2.2. Content-Encryption Key Derivation . . . . . . . . . . . . 6
2.3. Nonce Derivation . . . . . . . . . . . . . . . . . . . . 6
3. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Encryption of a Response . . . . . . . . . . . . . . . . 7
3.2. Encryption with Multiple Records . . . . . . . . . . . . 8
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
4.1. Automatic Decryption . . . . . . . . . . . . . . . . . . 9
4.2. Message Truncation . . . . . . . . . . . . . . . . . . . 9
4.3. Key and Nonce Reuse . . . . . . . . . . . . . . . . . . . 9
4.4. Data Encryption Limits . . . . . . . . . . . . . . . . . 10
4.5. Content Integrity . . . . . . . . . . . . . . . . . . . . 10
4.6. Leaking Information in Header Fields . . . . . . . . . . 10
4.7. Poisoning Storage . . . . . . . . . . . . . . . . . . . . 11
4.8. Sizing and Timing Attacks . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
5.1. The "aes128gcm" HTTP Content Coding . . . . . . . . . . . 12
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
6.1. Normative References . . . . . . . . . . . . . . . . . . 12
6.2. Informative References . . . . . . . . . . . . . . . . . 13
Appendix A. JWE Mapping . . . . . . . . . . . . . . . . . . . . 15
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 16
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
It is sometimes desirable to encrypt the contents of an HTTP message
(request or response) so that when the payload is stored (e.g., with
an HTTP PUT), only someone with the appropriate key can read it.
For example, it might be necessary to store a file on a server
without exposing its contents to that server. Furthermore, that same
file could be replicated to other servers (to make it more resistant
to server or network failure), downloaded by clients (to make it
available offline), etc., without exposing its contents.
These uses are not met by the use of Transport Layer Security (TLS)
[RFC5246], since it only encrypts the channel between the client and
server.
This document specifies a content coding (see Section 3.1.2 of
[RFC7231]) for HTTP to serve these and other use cases.
This content coding is not a direct adaptation of message-based
encryption formats -- such as those that are described by [RFC4880],
[RFC5652], [RFC7516], and [XMLENC]. Those formats are not suited to
stream processing, which is necessary for HTTP. The format described
here follows more closely to the lower-level constructs described in
[RFC5116].
To the extent that message-based encryption formats use the same
primitives, the format can be considered to be a sequence of
encrypted messages with a particular profile. For instance,
Appendix A explains how the format is congruent with a sequence of
JSON Web Encryption [RFC7516] values with a fixed header.
This mechanism is likely only a small part of a larger design that
uses content encryption. How clients and servers acquire and
identify keys will depend on the use case. In particular, a key
management system is not described.
1.1. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The "aes128gcm" HTTP Content Coding
The "aes128gcm" HTTP content coding indicates that a payload has been
encrypted using Advanced Encryption Standard (AES) in Galois/Counter
Mode (GCM) as identified as AEAD_AES_128_GCM in [RFC5116],
Section 5.1. The AEAD_AES_128_GCM algorithm uses a 128-bit content-
encryption key.
Using this content coding requires knowledge of a key. How this key
is acquired is not defined in this document.
The "aes128gcm" content coding uses a single fixed set of encryption
primitives. Cipher agility is achieved by defining a new content-
coding scheme. This ensures that only the HTTP Accept-Encoding
header field is necessary to negotiate the use of encryption.
The "aes128gcm" content coding uses a fixed record size. The final
encoding consists of a header (see Section 2.1) and zero or more
fixed-size encrypted records; the final record can be smaller than
the record size.
The record size determines the length of each portion of plaintext
that is enciphered. The record size ("rs") is included in the
content-coding header (see Section 2.1).
+-----------+ content
| data | any length up to rs-17 octets
+-----------+
|
v
+-----------+-----+ add a delimiter octet (0x01 or 0x02)
| data | pad | then 0x00-valued octets to rs-16
+-----------+-----+ (or less on the last record)
|
v
+--------------------+ encrypt with AEAD_AES_128_GCM;
| ciphertext | final size is rs;
+--------------------+ the last record can be smaller
AEAD_AES_128_GCM produces ciphertext 16 octets longer than its input
plaintext. Therefore, the unencrypted content of each record is
shorter than the record size by 16 octets. Valid records always
contain at least a padding delimiter octet and a 16-octet
authentication tag.
Each record contains a single padding delimiter octet followed by any
number of zero octets. The last record uses a padding delimiter
octet set to the value 2, all other records have a padding delimiter
octet value of 1.
On decryption, the padding delimiter is the last non-zero-valued
octet of the record. A decrypter MUST fail if the record contains no
non-zero octet. A decrypter MUST fail if the last record contains a
padding delimiter with a value other than 2 or if any record other
than the last contains a padding delimiter with a value other than 1.
The nonce for each record is a 96-bit value constructed from the
record sequence number and the input-keying material. Nonce
derivation is covered in Section 2.3.
The additional data passed to each invocation of AEAD_AES_128_GCM is
a zero-length octet sequence.
A consequence of this record structure is that range requests
[RFC7233] and random access to encrypted payload bodies are possible
at the granularity of the record size. Partial records at the ends
of a range cannot be decrypted. Thus, it is best if range requests
start and end on record boundaries. However, note that random access
to specific parts of encrypted data could be confounded by the
presence of padding.
Selecting the record size most appropriate for a given situation
requires a trade-off. A smaller record size allows decrypted octets
to be released more rapidly, which can be appropriate for
applications that depend on responsiveness. Smaller records also
reduce the additional data required if random access into the
ciphertext is needed.
Applications that don't depend on streaming, random access, or
arbitrary padding can use larger records, or even a single record. A
larger record size reduces processing and data overheads.
2.1. Encryption Content-Coding Header
The content coding uses a header block that includes all parameters
needed to decrypt the content (other than the key). The header block
is placed in the body of a message ahead of the sequence of records.
+-----------+--------+-----------+---------------+
| salt (16) | rs (4) | idlen (1) | keyid (idlen) |
+-----------+--------+-----------+---------------+
salt: The "salt" parameter comprises the first 16 octets of the
"aes128gcm" content-coding header. The same "salt" parameter
value MUST NOT be reused for two different payload bodies that
have the same input-keying material; generating a random salt for
every application of the content coding ensures that content-
encryption key reuse is highly unlikely.
rs: The "rs" or record size parameter contains an unsigned 32-bit
integer in network byte order that describes the record size in
octets. Note that it is, therefore, impossible to exceed the
2^36-31 limit on plaintext input to AEAD_AES_128_GCM. Values
smaller than 18 are invalid.
idlen: The "idlen" parameter is an unsigned 8-bit integer that
defines the length of the "keyid" parameter.
keyid: The "keyid" parameter can be used to identify the keying
material that is used. This field is the length determined by the
"idlen" parameter. Recipients that receive a message are expected
to know how to retrieve keys; the "keyid" parameter might be input
to that process. A "keyid" parameter SHOULD be a UTF-8-encoded
[RFC3629] string, particularly where the identifier might need to
be rendered in a textual form.
2.2. Content-Encryption Key Derivation
In order to allow the reuse of keying material for multiple different
HTTP messages, a content-encryption key is derived for each message.
The content-encryption key is derived from the "salt" parameter using
the HMAC-based key derivation function (HKDF) described in [RFC5869]
using the SHA-256 hash algorithm [FIPS180-4].
The value of the "salt" parameter is the salt input to the HKDF. The
keying material identified by the "keyid" parameter is the input-
keying material (IKM) to HKDF. Input-keying material is expected to
be provided to recipients separately. The extract phase of HKDF,
therefore, produces a pseudorandom key (PRK) as follows:
PRK = HMAC-SHA-256 (salt, IKM)
The info parameter to HKDF is set to the ASCII-encoded string
"Content-Encoding: aes128gcm" and a single zero octet:
cek_info = "Content-Encoding: aes128gcm" || 0x00
Note(1): Concatenation of octet sequences is represented by the "||"
operator.
Note(2): The strings used here and in Section 2.3 do not include a
terminating 0x00 octet, as is used in some programming languages.
AEAD_AES_128_GCM requires a 16-octet (128-bit) content-encryption key
(CEK), so the length (L) parameter to HKDF is 16. The second step of
HKDF can, therefore, be simplified to the first 16 octets of a single
HMAC:
CEK = HMAC-SHA-256(PRK, cek_info || 0x01)
2.3. Nonce Derivation
The nonce input to AEAD_AES_128_GCM is constructed for each record.
The nonce for each record is a 12-octet (96-bit) value that is
derived from the record sequence number, input-keying material, and
"salt" parameter.
The input-keying material and "salt" parameter are input to HKDF with
different info and length (L) parameters.
The length (L) parameter is 12 octets. The info parameter for the
nonce is the ASCII-encoded string "Content-Encoding: nonce",
terminated by a single zero octet:
nonce_info = "Content-Encoding: nonce" || 0x00
The result is combined with the record sequence number -- using
exclusive or -- to produce the nonce. The record sequence number
(SEQ) is a 96-bit unsigned integer in network byte order that starts
at zero.
Thus, the final nonce for each record is a 12-octet value:
NONCE = HMAC-SHA-256(PRK, nonce_info || 0x01) XOR SEQ
This nonce construction prevents removal or reordering of records.
3. Examples
This section shows a few examples of the encrypted-content coding.
Note: All binary values in the examples in this section use base64
encoding with URL and filename safe alphabet [RFC4648]. This
includes the bodies of requests. Whitespace and line wrapping is
added to fit formatting constraints.
3.1. Encryption of a Response
Here, a successful HTTP GET response has been encrypted. This uses a
record size of 4096 octets and no padding (just the single-octet
padding delimiter), so only a partial record is present. The input-
keying material is identified by an empty string (that is, the
"keyid" field in the header is zero octets in length).
The encrypted data in this example is the UTF-8-encoded string "I am
the walrus". The input-keying material is the value "yqdlZ-
tYemfogSmv7Ws5PQ" (in base64url). The 54-octet content body contains
a single record and is shown here using 71 base64url characters for
presentation reasons.
HTTP/1.1 200 OK
Content-Type: application/octet-stream
Content-Length: 54
Content-Encoding: aes128gcm
I1BsxtFttlv3u_Oo94xnmwAAEAAA-NAVub2qFgBEuQKRapoZu-IxkIva3MEB1PD-
ly8Thjg
Note that the media type has been changed to "application/octet-
stream" to avoid exposing information about the content.
Alternatively (and equivalently), the Content-Type header field can
be omitted.
Intermediate values for this example (all shown using base64url):
salt (from header) = I1BsxtFttlv3u_Oo94xnmw
PRK = zyeH5phsIsgUyd4oiSEIy35x-gIi4aM7y0hCF8mwn9g
CEK = _wniytB-ofscZDh4tbSjHw
NONCE = Bcs8gkIRKLI8GeI8
unencrypted data = SSBhbSB0aGUgd2FscnVzAg
3.2. Encryption with Multiple Records
This example shows the same message with input-keying material of
"BO3ZVPxUlnLORbVGMpbT1Q". In this example, the plaintext is split
into records of 25 octets each (that is, the "rs" field in the header
is 25). The first record includes one 0x00 padding octet. This
means that there are 7 octets of message in the first record and 8 in
the second. A key identifier of the UTF-8-encoded string "a1" is
also included in the header.
HTTP/1.1 200 OK
Content-Length: 73
Content-Encoding: aes128gcm
uNCkWiNYzKTnBN9ji3-qWAAAABkCYTHOG8chz_gnvgOqdGYovxyjuqRyJFjEDyoF
1Fvkj6hQPdPHI51OEUKEpgz3SsLWIqS_uA
4. Security Considerations
This mechanism assumes the presence of a key management framework
that is used to manage the distribution of keys between valid senders
and receivers. Defining key management is part of composing this
mechanism into a larger application, protocol, or framework.
Implementation of cryptography -- and key management in particular --
can be difficult. For instance, implementations need to account for
the potential for exposing keying material on side channels, such as
might be exposed by the time it takes to perform a given operation.
The requirements for a good implementation of cryptographic
algorithms can change over time.
4.1. Automatic Decryption
As a content coding, a "aes128gcm" content coding might be
automatically removed by a receiver in a way that is not obvious to
the ultimate consumer of a message. Recipients that depend on
content-origin authentication using this mechanism MUST reject
messages that don't include the "aes128gcm" content coding.
4.2. Message Truncation
This content encoding is designed to permit the incremental
processing of large messages. It also permits random access to
plaintext in a limited fashion. The content encoding permits a
receiver to detect when a message is truncated.
A partially delivered message MUST NOT be processed as though the
entire message was successfully delivered. For instance, a partially
delivered message cannot be cached as though it were complete.
An attacker might exploit willingness to process partial messages to
cause a receiver to remain in a specific intermediate state.
Implementations performing processing on partial messages need to
ensure that any intermediate processing states don't advantage an
attacker.
4.3. Key and Nonce Reuse
Encrypting different plaintext with the same content-encryption key
and nonce in AES-GCM is not safe [RFC5116]. The scheme defined here
uses a fixed progression of nonce values. Thus, a new content-
encryption key is needed for every application of the content coding.
Since input-keying material can be reused, a unique "salt" parameter
is needed to ensure that a content-encryption key is not reused.
If a content-encryption key is reused -- that is, if input-keying
material and "salt" parameter are reused -- this could expose the
plaintext and the authentication key, nullifying the protection
offered by encryption. Thus, if the same input-keying material is
reused, then the "salt" parameter MUST be unique each time. This
ensures that the content-encryption key is not reused. An
implementation SHOULD generate a random "salt" parameter for every
message.
4.4. Data Encryption Limits
There are limits to the data that AEAD_AES_128_GCM can encipher. The
maximum value for the record size is limited by the size of the "rs"
field in the header (see Section 2.1), which ensures that the 2^36-31
limit for a single application of AEAD_AES_128_GCM is not reached
[RFC5116]. In order to preserve a 2^-40 probability of
indistinguishability under chosen plaintext attack (IND-CPA), the
total amount of plaintext that can be enciphered with the key derived
from the same input-keying material and salt MUST be less than 2^44.5
blocks of 16 octets [AEBounds].
If the record size is a multiple of 16 octets, this means that 398
terabytes can be encrypted safely, including padding and overhead.
However, if the record size is not a multiple of 16 octets, the total
amount of data that can be safely encrypted is reduced because
partial AES blocks are encrypted. The worst case is a record size of
18 octets, for which at most 74 terabytes of plaintext can be
encrypted, of which at least half is padding.
4.5. Content Integrity
This mechanism only provides content-origin authentication. The
authentication tag only ensures that an entity with access to the
content-encryption key produced the encrypted data.
Any entity with the content-encryption key can, therefore, produce
content that will be accepted as valid. This includes all recipients
of the same HTTP message.
Furthermore, any entity that is able to modify both the Content-
Encoding header field and the HTTP message body can replace the
contents. Without the content-encryption key or the input-keying
material, modifications to, or replacement of, parts of a payload
body are not possible.
4.6. Leaking Information in Header Fields
Because only the payload body is encrypted, information exposed in
header fields is visible to anyone who can read the HTTP message.
This could expose side-channel information.
For example, the Content-Type header field can leak information about
the payload body.
There are a number of strategies available to mitigate this threat,
depending upon the application's threat model and the users'
tolerance for leaked information:
1. Determine that it is not an issue. For example, if it is
expected that all content stored will be "application/json", or
another very common media type, exposing the Content-Type header
field could be an acceptable risk.
2. If it is considered sensitive information and it is possible to
determine it through other means (e.g., out of band, using hints
in other representations, etc.), omit the relevant headers, and/
or normalize them. In the case of Content-Type, this could be
accomplished by always sending Content-Type:
application/octet-stream (the most generic media type), or no
Content-Type at all.
3. If it is considered sensitive information and it is not possible
to convey it elsewhere, encapsulate the HTTP message using the
application/http media type (see Section 8.3.2 of [RFC7230]),
encrypting that as the payload of the "outer" message.
4.7. Poisoning Storage
This mechanism only offers data-origin authentication; it does not
perform authentication or authorization of the message creator, which
could still need to be performed (e.g., by HTTP authentication
[RFC7235]).
This is especially relevant when an HTTP PUT request is accepted by a
server without decrypting the payload; if the request is
unauthenticated, it becomes possible for a third party to deny
service and/or poison the store.
4.8. Sizing and Timing Attacks
Applications using this mechanism need to be aware that the size of
encrypted messages, as well as their timing, HTTP methods, URIs and
so on, may leak sensitive information. See, for example, [NETFLIX]
or [CLINIC].
This risk can be mitigated through the use of the padding that this
mechanism provides. Alternatively, splitting up content into
segments and storing them separately might reduce exposure. HTTP/2
[RFC7540] combined with TLS [RFC5246] might be used to hide the size
of individual messages.
Developing a padding strategy is difficult. A good padding strategy
can depend on context. Common strategies include padding to a small
set of fixed lengths, padding to multiples of a value, or padding to
powers of 2. Even a good strategy can still cause size information
to leak if processing activity of a recipient can be observed. This
is especially true if the trailing records of a message contain only
padding. Distributing non-padding data across records is recommended
to avoid leaking size information.
5. IANA Considerations
5.1. The "aes128gcm" HTTP Content Coding
This memo registers the "aes128gcm" HTTP content coding in the "HTTP
Content Coding Registry", as detailed in Section 2.
o Name: aes128gcm
o Description: AES-GCM encryption with a 128-bit content-encryption
key
o Reference: this specification
6. References
6.1. Normative References
[FIPS180-4]
National Institute of Standards and Technology, "Secure
Hash Standard (SHS)", FIPS PUB 180-4,
DOI 10.6028/NIST.FIPS180-4, August 2015,
<http://nvlpubs.nist.gov/nistpubs/FIPS/
NIST.FIPS.180-4.pdf>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <http://www.rfc-editor.org/info/rfc3629>.
[RFC5116] McGrew, D., "An Interface and Algorithms for Authenticated
Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
<http://www.rfc-editor.org/info/rfc5116>.
[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
DOI 10.17487/RFC5869, May 2010,
<http://www.rfc-editor.org/info/rfc5869>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing",
RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>.
[RFC7231] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Semantics and Content", RFC 7231,
DOI 10.17487/RFC7231, June 2014,
<http://www.rfc-editor.org/info/rfc7231>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <http://www.rfc-editor.org/info/rfc8174>.
6.2. Informative References
[AEBounds] Luykx, A. and K. Paterson, "Limits on Authenticated
Encryption Use in TLS", March 2016,
<http://www.isg.rhul.ac.uk/~kp/TLS-AEbounds.pdf>.
[CLINIC] Miller, B., Huang, L., Joseph, A., and J. Tygar, "I Know
Why You Went to the Clinic: Risks and Realization of HTTPS
Traffic Analysis", DOI 10.1007/978-3-319-08506-7_8, March
2014, <https://arxiv.org/abs/1403.0297>.
[NETFLIX] Reed, A. and M. Kranch, "Identifying HTTPS-Protected
Netflix Videos in Real-Time", Proceedings of the Seventh
ACM on Conference on Data and Application Security and
Privacy CODASPY '17, DOI 10.1145/3029806.3029821, 2017.
[RFC4648] Josefsson, S., "The Base16, Base32, and Base64 Data
Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006,
<http://www.rfc-editor.org/info/rfc4648>.
[RFC4880] Callas, J., Donnerhacke, L., Finney, H., Shaw, D., and R.
Thayer, "OpenPGP Message Format", RFC 4880,
DOI 10.17487/RFC4880, November 2007,
<http://www.rfc-editor.org/info/rfc4880>.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246,
DOI 10.17487/RFC5246, August 2008,
<http://www.rfc-editor.org/info/rfc5246>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<http://www.rfc-editor.org/info/rfc5652>.
[RFC7233] Fielding, R., Ed., Lafon, Y., Ed., and J. Reschke, Ed.,
"Hypertext Transfer Protocol (HTTP/1.1): Range Requests",
RFC 7233, DOI 10.17487/RFC7233, June 2014,
<http://www.rfc-editor.org/info/rfc7233>.
[RFC7235] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Authentication", RFC 7235,
DOI 10.17487/RFC7235, June 2014,
<http://www.rfc-editor.org/info/rfc7235>.
[RFC7516] Jones, M. and J. Hildebrand, "JSON Web Encryption (JWE)",
RFC 7516, DOI 10.17487/RFC7516, May 2015,
<http://www.rfc-editor.org/info/rfc7516>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>.
[XMLENC] Eastlake, D., Reagle, J., Hirsch, F., and T. Roessler,
"XML Encryption Syntax and Processing Version 1.1", World
Wide Web Consortium Recommendation
REC-xmlenc-core1-20130411, April 2013,
<http://www.w3.org/TR/2013/REC-xmlenc-core1-20130411>.
Appendix A. JWE Mapping
The "aes128gcm" content coding can be considered as a sequence of
JSON Web Encryption (JWE) [RFC7516] objects, each corresponding to a
single fixed-size record that includes trailing padding. The
following transformations are applied to a JWE object that might be
expressed using the JWE Compact Serialization:
o The JWE Protected Header is fixed to the value { "alg": "dir",
"enc": "A128GCM" }, describing direct encryption using AES-GCM
with a 128-bit content-encryption key. This header is not
transmitted, it is instead implied by the value of the Content-
Encoding header field.
o The JWE Encrypted Key is empty, as stipulated by the direct
encryption algorithm.
o The JWE Initialization Vector ("iv") for each record is set to the
exclusive-or of the 96-bit record sequence number, starting at
zero, and a value derived from the input-keying material (see
Section 2.3). This value is also not transmitted.
o The final value is the concatenated header, JWE Ciphertext, and
JWE Authentication Tag, all expressed without base64url encoding.
The "." separator is omitted, since the length of these fields is
known.
Thus, the example in Section 3.1 can be rendered using the JWE
Compact Serialization as:
eyAiYWxnIjogImRpciIsICJlbmMiOiAiQTEyOEdDTSIgfQ..Bcs8gkIRKLI8GeI8.
-NAVub2qFgBEuQKRapoZuw.4jGQi9rcwQHU8P6XLxOGOA
Where the first line represents the fixed JWE Protected Header, an
empty JWE Encrypted Key, and the algorithmically determined JWE
Initialization Vector. The second line contains the encoded body,
split into JWE Ciphertext and JWE Authentication Tag.
Acknowledgements
Mark Nottingham was an original author of this document.
The following people provided valuable input: Richard Barnes, David
Benjamin, Peter Beverloo, JR Conlin, Mike Jones, Stephen Farrell,
Adam Langley, James Manger, John Mattsson, Julian Reschke, Eric
Rescorla, Jim Schaad, and Magnus Westerlund.
Author's Address
Martin Thomson
Mozilla
Email: martin.thomson@gmail.com

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